T-Mobile UK's Capped Data Rate actually is ...

Since WebNWalk [& later its offspring] was first launched some 5yrs odd years ago, Capped Data Rate has referred to the speed at which your data connection is throttled - not any volume usage limit [the latter having varied extensively from 10GB/3GB/2GB/1GB/750MB/500MB/50MB/ to 40MB dependant on tariff].

The standard handset connection has always been limited to 3G speeds, irrespective of whether there's hsdpa coverage].

The WebNWalkPro/Plus/MobileBroadbroadPlus tariffs were enabled with full available hspda [speeds of 3.6-4.5MBps d/ld & 1-2MB/s up/ld; irrespective of whether tethering handsets as modems, or just the [currently more usual] usb modems.

With the introduction of Android, the d/ld CPR for WebNWalkPlus variant handset tariffs was reduced to 1.8MB/s max, and at some point last year [I would hazard a guess to align with the introduction of iPhone to the network, though it may have been earlier] the up/ld data rate was throttled to 3G speeds of approx 384KB/s max.

I hope this provides some clarity.